1. Field of the Invention
This invention relates to a fluorescent substance of a Pr-activated rare earth oxysulfide, a method for the production thereof, a radiation detector using the fluorescent substance, and an X-ray CT apparatus. More particularly, this invention relates to a Pr-activated rare earth oxysulfide fluorescent substance possessed of an exalted capacity for light emission, a method for the production thereof, a radiation detector using the fluorescent substance, and an X-ray CT apparatus using the radiation detector.
2. Description of the Related Art
Heretofore, the practice of conducting a nondestructive inspection of a body under test by means of X-ray CT has been in vogue particularly in the medical field and the industrial field. In the X-ray CT apparatus to be employed for this inspection, the detector which utilizes a scintillator is used for detecting the radiation transmitted through a given body. The scintillator is a substance which, on being stimulated as by the X ray, emits a visible radiation or electromagnetic waves of a wavelength approximating that of a visible radiation. As materials usable effectively for this scintillator, single crystals such as NaI, CsI, and CdWO.sub.4, ceramics such as BaFCl:Eu, LaOBr:Tb, CsI:Tl, CaWO.sub.4, and CdWO.sub.4, the cubic rare earth oxide ceramics disclosed in JP-A-59-27,283, and the rare earth oxysulfide ceramics such as Gd.sub.2 O.sub.2 S:Pr disclosed in JP-A-58-204,088 have been known.
Among materials for the scintillator mentioned above, the rare earth oxysulfide ceramics such as Gd.sub.2 O.sub.2 S:Pr prove to be particularly suitable because of high luminous efficiency. The sulfidizing agent which is mixed with a rare earth oxide powder and the alkali metal salt of phosphoric acid which is used as a flux chiefly persist in the fluorescent substance and impede the improvement of the light transmission property of the rare earth oxysulfide ceramics.
For the purpose of solving this problem, JP-A-03-243,686, for example, discloses an invention which is aimed at enabling a fluorescent substance to retain a necessary sintering property intact and acquire improved crystallinity and light transmission property by limiting within specific ranges the amounts of alkali metals and phosphorus persisting in the fluorescent substance.
The fluorescent substance which is obtained as disclosed in JP-A-03-243,686, however, is still deficient in luminous emission intensity and is in need of further improvement.